The present invention relates to an etching solution for a titanium-based thin film and a process for manufacturing a semiconductor device using this etching solution. More particularly, the present invention concerns an etching solution and a manufacturing process when fine-pattern metal electrodes and metal wirings are formed by using a titanium-based thin film in a process of manufacturing a semiconductor device.
Conventionally, it is known that hydrofluoric acid-based solutions are generally used when titanium-based thin films are etched in the process of manufacturing semiconductor devices. For example, it is stated in Japanese Patent Unexamined Publication No. 124726/1984 that, in a case where platinum or gold is used as a wiring material, in pattering the material into a fine pattern, a titanium-based thin film is formed on a platinum or gold thin film, the titanium-based thin film is subjected to patterning by a photolithographic etching process using a resist, and etching is performed with aqua regia by using the patterned titanium-based thin film as a protective film, thereby forming a platinum or gold wiring pattern. The publication discloses that an about 5% hydrofluoric acid solution is used in etching the titanium-based thin film.
In addition, Japanese Patent Unexamined Publication No. 124726/1984 discloses that, in a case where a titanium-based thin film is formed as a protective metal layer for a wiring electrode formed of aluminum, in patterning the titanium-based thin film, since there arises a need to selectively etch the titanium-based thin film with respect to the lower aluminum layer, an ethylene diamine tetraacetic acid--NH.sub.4 OH--H.sub.2 O.sub.2 --H.sub.2 O-based etching solution is used.
However, if the aforementioned hydrofluoric acid-based etching solution is used, at a time when a titanium-based thin film having a thickness of 1000 angstroms or thereabouts is subjected to patterning by etching using a resist pattern 31 so as to form a metal electrode or a metal wiring pattern 32, as shown in FIG. 3(a), irregularities of 1 .mu.m or thereabouts occur on end faces 33 of the pattern formed by etching, and an amount of side etching at a portion below an end of the resist pattern 31 becomes large at 1 to 3 .mu.m or thereabouts. Hence, there has been a problem in that this process is unsuitable for patterning for fine processing.
Accordingly, in a case where a source electrode 41 and a drain electrode 42 of a thin-film transistor (TFT) is fabricated by using, for instance, a titanium-based thin film, if the amount of side etching is large, as shown in FIG. 3(b), it is impossible to effect patterning along the resist pattern 31. As a result, amounts of overlap 11 and 12 between the source electrode 41 side and the drain electrode 42 side on the one hand, and a channel protecting layer 44 on a channel layer (semiconductor active layer) 43 on the other, become nonuniform (defective overlaps), thereby resulting in variations in the on-state current of the TFTs.
In addition, according to Japanese Patent Unexamined Publication No. 138235/1980, the ethylene diamine tetraacetic acid--NH.sub.4 OH--H.sub.2 O.sub.2 --H.sub.2 O-based etching solution is used to take a selectivity ratio between a titanium thin film and an aluminum thin film which is at the lower layer when the aluminum thin film and the titanium thin film are laminated. However, in cases where the titanium thin film in the lower layer is a glass substrate, an SiN.sub.x film, an SiO.sub.2 film, or an a--Si:H film, such a material is unaffected by an NH.sub.4 OH-based etching solution, so that the ethylene diamine tetraacetic acid is not required in the etching solution.